High purity copper (Cu) of at least 99.999 wt. % purity (referred to as 5N Cu) is useful for producing high purity Cu interconnects for use in integrated circuits. The Cu purity of the resultant target is important to maintain the low resistivity of the Cu line. The interconnects can be created by sputtering 5N or higher purity Cu material from Cu target assemblies. The assembly can be typically prepared by diffusion bonding a high purity Cu target blank or disc to a high strength alloy backing plate. The finer grain size can improve Cu target sputter performance. For example, finer grain sizes can allow the target to sputter faster and may result in fewer particle inclusions incorporated into the deposited film. Finer grain size (e.g., 50 microns or less) can also improve resultant film properties, including the film resistivity (Rs) and thickness uniformity.
Technology advancements with integrated circuits are now driving the need for even higher purity Cu interconnects. Today's Cu interconnects require higher conductivity which is driving the need for targets of 99.9999 wt. % (6N Cu). The shift from 200 mm to 300 mm wafer necessitates 6N or higher Cu target assemblies to be significantly thicker and larger in diameter than precursor target assemblies previously employed to produce 200 mm wafers.
However, there remain design challenges for manufacturing 6N and higher Cu targets. In contrast to Cu target assemblies at a purity of 5N which contain more solutes that can effectively immobilize or pin the grain boundaries during annealing and recrystallization, the control of grain size and grain growth for 6N and higher Cu purity levels become exceedingly difficult, in part because the 6N and higher Cu purity levels cannot contain the same amount of alloying solutes as contained in the 5N Cu material. At these higher purity levels, copper is particularly susceptible to grain growth in which some grains that are scattered throughout the material grow faster than the matrix. As the grains grow, they consume smaller matrix grains until the entire structure results in a large grain size, which can be in excess of 250 microns (μm). To alleviate the grain growth problem, the high purity Cu targets can be manufactured below a critical temperature at which the onset of anomalous grain growth will not occur. However, typically the minimum temperature required for diffusion bonding to produce adequate bond strength at the target-backing plate interface is higher than the critical temperature. The bond strength must be sufficiently high such that debonding of the target from the backing plate does not occur during target sputtering. As sputtering power levels continue to increase, the need for higher bond strengths becomes more critical. Furthermore, higher power sputtering conditions generate considerable heat to cause localized grain growth at the sputtering surface of the target.
Generally speaking, unlike 5N Cu target assemblies, the ability for 6N- or higher purity Cu target assemblies to achieve and maintain fine grain size along with adequate bond strength are competing design attributes. In other words, acceptable fine grain size cannot be achieved at the expense of lower and unacceptable bond strength and vice versa at 6N or higher purity Cu target assemblies. Higher temperatures involved in diffusion bonding change the microstructure obtained during pre-bonding processing. Even if grain size and the desired random grain orientation can be achieved during manufacture of the target blank, the attributes are lost by current-diffusion bonding techniques. In fact, diffusion bonding of 6N and higher purity Cu target blanks can nearly double the grain size.
Several high purity Cu targets are currently available that attempt to offer fine grain size without incurring loss of bond strength. For example, monolithic 6N Cu targets are currently utilized. Monolithic target as recognized in the industry refers to the target not being affixed to a backing plate. In this manner, the design challenges of unacceptable bond strength are eliminated, thereby allowing the target blank to be manufactured at lower temperatures to prevent grain growth. However, the absence of a high strength backing plate means that the target can bow and cause warpage during sputtering. The problem is compounded with 6N and higher purity Cu which is being sputtered at increasingly higher power levels. Such monolithic sputter targets without backing plates become less feasible in view of the continuing increase in target diameters required for sputtering larger size wafers. As a result, 6N or higher purity monolithic Cu targets are not viable for today's demanding sputtering applications.
Several 6N or higher purity copper target-backing plate assemblies are available. However, none are inadequate. For instance, micro-alloy additions to Cu targets exceeding 99.999 wt. % have been employed in an effort to maintain and stabilize the fine-grained microstructure during elevated temperatures of diffusion bonding and subsequent sputter power levels. Although the micro-alloy additions can allow diffusion bonding at elevated temperatures to occur without grain growth at the elevated temperatures, the additions are problematic because they are a source of undesirable contaminant which is now introduced into the target. The sputter process may cause the micro-alloy additions to become incorporated into the resultant film that is deposited. Additionally, several end-use applications require utilizing target assemblies at 6N or higher purity Cu that cannot tolerate any amount of alloying introduced therein. Accordingly, the need for 6N or higher purity Cu target assemblies without alloying elements has proven challenging, as there is no resistance to movement of the grain boundaries to form large grains at the higher temperatures.
In an effort to maintain 6N or higher purity Cu target assemblies without micro-alloying, the bonding temperature must be reduced. In this regard, several high purity Cu target assemblies are available that utilize an interlayer to provide the necessary bond strength. For instance, silver interlayers are commonly utilized at the interface as part of the bonding process. However, the temperature that is required for use of a silver interlayer is sufficiently high to cause localized grain coarsening at the bond interface. Loss of microstructure control can occur. The problem also occurs with other interlayers such that the temperatures required for their use have proven too high to maintain grain structure stability.
Soldering the target to the backing plate has also been utilized, particularly those solder materials having lower melting points. However, solder bonds have proven weak and are susceptible to debonding during the sputtering operation. Furthermore, the relatively lower temperatures associated with lower melting point solders reduces the target's temperature range for sputtering. Thus, solder-bonded assemblies can only be operated at a lower power level to prevent separation of the 6N or higher purity Cu target from the backing plate. This is problematic because sputtering at reduced power levels decreases the sputtering rate.
Grooved interfaces have been used with the design objective of producing a mechanically interlocked interface which can achieve adequate bond strength without the onset of grain growth. However, grooved target interfaces contain voids at the interface that can result in poor sputtering performance as well as less material utilization of the target. Additionally, the grooves may degrade during high pressure bonding of the assembly, thereby adversely affecting the structural integrity of the bonded structure, and potentially shortening the effective lifetime of the target.
In view of the drawbacks with currently available 6N or higher purity Cu sputter targets, there is a growing need for 6N or higher purity non-alloyed Cu targets to retain fine microstructure without losing bond strength in the production of 300 mm Cu coated wafers.